Apparatus for driving and controlling synchronous motor using permanent magnets as its field system

Abstract: When a rotational speed (N) of a synchronous motor (14) using permanent magnets as its field system is equal to or higher than a base rotational speed (N B ), a current condition computing section (16) computes a field weakening reference current (I d *) in accordance with a battery voltage (V dc ), reference torque (T*), and the rotational speed (N). An inverter (12) performs the field weakening control of the synchronous motor (14) in accordance with the field weakening reference current (I d *) sent from the current condition computing section (16) to prevent the deterioration of efficiency and the deficiency of output due to excessive or insufficient field weakening current (I d ). The current condition computing section (16) determines an optimum reference current (I d *, I q *) by using a converged battery voltage (V S ) when the battery voltage (V dc ) is converged after initial control.

Description: The present invention relates to a driving and controlling apparatus for controlling the output torque of a synchronous motor using permanent magnets as its field system (hereafter referred to as PM motor) so as to attain a reference torque by controlling the vector of a motor current. Particularly, it relates to the control of a PM motor which can be suitably mounted on an electric vehicle and can execute field weakening control. A PM motor is a synchronous motor using permanent magnets as its field system. Therefore, it is characterized by having a large magnetomotive force per unit volume. This characteristic is effective for increasing the output of a motor and reducing the size of a motor. Particularly, the PM motor is preferable as the drive motor of an electric vehicle. FIG. 20 shows an equivalent circuit per phase of a PM motor. In FIG. 20, the magnetomotive force of permanent magnets used as a field system, that is, the main magnetic flux of the PM motor, is shown by the symbol E 0 . When assuming the axial angular velocity of the PM motor as ω (=2πN, N: rotational speed of the motor), the counter electromotive force generated during operation of the PM motor can be expressed as ωE o . When the PM motor is driven by converting the discharge output of a battery into alternating current by an inverter and using the AC power, the terminal voltage V of the PM motor is obtained by multiplying a battery voltage V dc by the voltage conversion ratio of the inverter. In FIG. 20, symbol R represents the primary resistance per phase of the PM motor, L represents the inductance per phase, and I represents the primary current (phase current). The motor current I of the PM motor can be decomposed into the vector components, i.e., the field current I d and the torque current I q . The field current I d is a component for generating a field flux in the PM motor and the torque current I q is a component for generating torque by intersecting with the field flux of permanent magnets, that is, the main magnetic flux E 0 . When the PM motor is provided with saliency, the field current I d also generates torque. The torque generated when the torque current I q intersects with the main magnetic flux E 0 is known as magnetic torque and the torque caused by the saliency is known as reluctance torque. The following equation (1) is an expression showing the torque T of the PM motor, in which the first term at the right side shows magnetic torque and the second term shows reluctance torque. Therefore, the second term does not appear when the PM motor is a non-salient-pole motor. T = E 0 + (L d - L q )I d I q L d and L q in the equation (1) are called d-axis inductance and q-axis inductance, respectively, which are the d-axis component and q-axis component of the inductance L of the PM motor, respectively. The terminal voltage V of the PM motor can also be decomposed into the d-axis component and q-axis component. The d-axis component and q-axis component of the terminal voltage V, that is, d-axis voltage V d and q-axis voltage V q , can be expressed as shown below by using the field current I d , torque current I q , d-axis inductance L d , and q-axis inductance L q when the number of pole pairs is 1. V d = RI d + jωL q I q V q = jωL d I d + RI q + ωE 0 j: An imaginary component Moreover, the following relationship exists between d-axis voltage V d and q-axis voltage V q . |V| 2 = |V d | 2 + |V q | 2 In general, because the primary resistance R of the PM motor is much smaller than the inductance, it is possible to ignore the resistance R. When the primary resistance R is ignored, the equation (2) is expressed as shown below. FIG. 21 shows a vector diagram of the PM motor drawn in accordance with the equation (4). V d = jωL q I q V q = jωL d I d + ωE 0 V = ωE 0 + jωL q I q + jωL d I d To control the PM motor having the above characteristics, it is preferable to control the vector of the motor current I. For example, it is preferable to decompose the motor current I into the field current component I d and the torque current component I q and control the absolute value of each component. To control the vector of the motor current I of the PM motor, the controller receives a command for a vector to be output from the PM motor, that is, a reference torque T * from a supervisor controller. The controller receiving the reference torque T * determines the field current I d and the torque current I q which are control targets so that the reference torque T * is realized. In this case, it is not necessary to excite the PM motor with the field current I d , because permanent magnets generate the main magnetic flux. Therefore, the field current I d is normally controlled to 0 or some other specified value. When the field current I d is kept constant, it is possible to control the torque T with the torque current I q as shown by the equation (1). Therefore, the reference torque T* is exclusively used to determine the torque current I q . The controller outputs the determined field current I d and torque current I q to a motor controller as a reference field current I d * and a reference torque current I q *, respectively. The motor controller controls a power converter such as an inverter so that the field current I d and the torque current I q corresponding to the reference field current I d * and the reference torque current I q *, respectively, flow through the PM motor. Though the motor current I can be decomposed into the vector components of the field current I d and the torque current I q as shown above, it is also possible to decompose the motor current I into an absolute value component |I| and a phase component argI. To control the vector of the current I of the PM motor, it is preferable to perform the field weakening control at the same time. The field weakening control is defined as the control for weakening a field flux when the PM motor rotates at a high speed. As shown in the equation (4) and FIG. 21, when the rotational speed N of the PM motor, namely, the axial angular velocity ω of the PM motor increases, the counter electromotive force ωE 0 increases in proportion to the increase of the axial angular velocity ω and the terminal voltage V of the PM motor increases with the increase of the counter electromotive force ωE 0 . When the terminal voltage V of the PM motor exceeds a value corresponding to the battery voltage V dc , a voltage corresponding to the difference between the terminal voltage V and the battery voltage V dc is applied to an inverter provided between the PM motor or the power source. This voltage causes the inverter or the like to be damaged. Hereafter, the rotational speed N of the PM motor when the terminal voltage V equals a value corresponding to the battery voltage V dc is referred to as a base rotational speed N B . The region of rotational speed equal to or higher than the base rotational speed N B is referred to as a high rotation region, the region of rotational speed equal to or lower than the base rotational speed N B is referred to as a low rotation region, and the region of rotational speed close to the base rotational speed N B is referred to as a medium rotation region. In the PM motor, a field flux is mainly generated by permanent magnets. However, when the vector of the motor current I is controlled as described above, it is possible to generate a field flux having a necessary intensity by utilizing the field current I d , and also to generate a field flux with an intensity which partially cancels the field flux (main magnetic flux) E 0 generated by the permanent magnets. The field weakening control can specifically be executed as the control for generating a field flux with an intensity which partially cancels the main magnetic flux E 0 , by controlling the field current I d to a negative value. When this type of control is performed, the absolute value of the terminal voltage V decreases as shown by the equation (4) and FIG. 21. Therefore, when performing the field weakening control by aiming at the field current I d with a proper intensity, it is possible to control the terminal voltage V in the high rotation region to the battery voltage V dc or lower. The field current I d when the field weakening control is executed is referred to as a field weakening current. To execute the field weakening control, the value of the field weakening current I d is normally determined in accordance with the rotational speed N of the PM motor and the requested output torque (= reference torque T*) by assuming that the battery voltage V dc is constant. For example, as shown in FIG. 22, the field current I d is controlled to 0 in the low rotation region. As the rotational speed N increases exceeding the base rotational speed N B , the effective amplitude of the field weakening current I d is varied from -30 A to -50 A, to -100 A,...in that order. The field weakening control is disclosed in "Variable Speed Drive System of Permanent Magnet Synchronous Motors with Flux-weakening Control", Keita HATANAKA et al., at the 1991 General Conference No. 74 of the Industrial Application Division of The Institute of Electrical Engineers of Japan, pp 310-315. However, because the actual battery voltage V dc greatly depends on its state of charge (SOC) and load state, the field weakening current I d may become too large or too small. This causes a problem that the efficiency lowers or the reference torque T* is not accurately realized. In this case, a value obtained by converting the battery voltage V dc , assumed to determine the field weakening current I d , into a value comparable with the terminal voltage V is referred to as an allowable power-source voltage V B . When the allowable power-source voltage V B is higher than the actual battery voltage V dc , the field weakening current I d with an absolute value excessively larger than reference torque T * flows. In other words, the absolute value of the field current I d increases, which is a current component not contributing to generation of torque. This means that the efficiency lowers. Conversely, when the allowable power-source voltage V B is lower than the actual battery voltage V dc , it is impossible to provide sufficient field weakening current I d to control the terminal voltage V to the allowable power-source voltage V B or lower, or sufficient torque current I q to realize the reference torque T*. Document "IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL", vol. 35, no. 4, November 1988, New York US, pages 544 - 559, B.K. BOSE ET ALK "A Microcomputer-Based Control and Simulation of an advanced IPM Synchronous Machine Drive for Electric Propulsion" discloses a simulation system of a digitally controlled IPM synchronous machine which constitutes part of an electric vehicle drive system, wherein the synchronous machine (synchronous motor) is provided with permanent magnets as its field system and is supplied with a necessary drive current by a power source in the form of a battery. A control means (inverter) controllably converts the d.c. voltage of the battery into a variable-frequency variable-voltage alternating current as a power supply for the synchronous machine, whereby the drive system is operable in all four quadrants and regenerative braking is performed by returning the generated energy back to the battery for recharging the battery. When the synchronous machine reaches higher speeds causing a dominant armature reaction effect, the drive system enters into a field weakening control mode thereby defining a field weakening current component for partially cancelling the field flux provided by the permanent magnets. This field weakening control is provided on the basis of the monitored rotational speed of the synchronous machine and further predetermined operating conditions. Furthermore, reference values of control parameters are determined by simulation process (calculations and estimations), which enables the control of the synchronous machine by feedback signal processing. However, when carrying out the field weakening control in the above driving system, the varying battery voltage is not taken into consideration. It is the first object of the present invention to make it possible to control the field weakening current I d in accordance with a change of the actual battery voltage V dc and to execute optimum field weakening control. It is the second object of the present invention to secure the efficiency and favorably achieve the reference torque T*. It is the third object of the present invention to secure the efficiency and achieve the reference torque T* while considering the operation of an inverter, when one is used. It is the fourth object of the present invention to secure the accelerating performance from standstill and to simultaneously prevent the efficiency from lowering due to decrease of the battery voltage V dc caused by degradation of SOC (relative remaining capacity) of the battery. It is the fifth object of the present invention to improve the drive feeling of an electric vehicle. It is the sixth object of the present invention to allow a user to select the drive in consideration of efficiency in consideration of output, and thus, it is possible to flexibly fulfill the user's request. It is the seventh object of the present invention to favorably secure the output of a loaded motor. It is the eighth object of the present invention to realize an efficient control performance. It is the ninth object of the present invention to minimize the storage capacity necessary for control. According to the present invention the above objects are accomplished by a driving and controlling apparatus as claimed in the appended independent claim 1, and in particular, according to a first embodiment of the present invention by a driving and controlling apparatus for use in a drive unit having a synchronous motor using permanent magnets as its field system which can be excited by a field flux generated by a field current component in a primary current; a battery as a source of power; and power conversion means for converting power discharged from the battery into alternating current and feeding the alternating current as the primary current to the synchronous motor; said apparatus comprising: current control means for controlling the primary current of the synchronous motor in accordance with a reference current; and current condition determination means for carrying out a field weakening being characterized in that said current condition determination means is provided for minimizing the reference current in accordance with a voltage of the battery under predetermined conditions and at least partially cancelling the field flux generated by the permanent magnet with the field flux generated by the field current component so that a counter electromotive force of the synchronous motor does not exceed the voltage of the battery and so that the field flux generated by the field current component varies as the voltage of the battery varies. Further embodiments of the present invention are recited in the appended subclaims. FIG. 1 is a block diagram showing the system constitution of an electric vehicle to which each embodiment of the present invention is suitable; FIG. 2 is a flow chart showing the operation of a current condition computing section in the first embodiment of the present invention; FIG. 3 is an illustration showing the relationship between field weakening current, terminal voltage, and efficiency converted into values per phase; FIG. 3A is an illustration showing the relationship between field weakening current, terminal voltage, maximum motor-applied voltage and motor rotational speed; FIG. 4 is an illustration conceptually showing the field current limiting method in the first embodiment, in which symbol N max represents the maximum rotational speed of a PM motor; FIG. 5 is an illustration conceptually showing the output limiting method in the first embodiment; FIG. 6 is an illustration conceptually showing an output limiting method according to the prior art; FIG. 7 is a timing chart showing a battery-voltage fluctuation due to loaded operation of a motor; FIG. 8 is an illustration showing the relation between motor rotational speed, reference torque value and battery voltage, and optimum field current component for maximizing the motor efficiency; FIG. 9 is an illustration showing the relation between motor rotational speed, reference torque value and battery voltage, and optimum torque current component for optimizing the motor efficiency about a salient-pole motor; FIG. 10 is an illustration showing the linear equation approximation of the relationship between optimum field current component and battery voltage; FIG. 11 is an illustration showing the linear equation approximation of the relationship between optimum torque current component and battery voltage; FIG. 12 is a flow chart showing the entire operation of the current condition computing section in the second embodiment of the present invention; FIG. 13 is a flow chart showing the reference current value correcting operation of the current condition computing section in the second embodiment; FIG. 14 is a timing chart showing the control timing in the second embodiment; FIG. 15 is a flow chart showing the reference minimum voltage determining operation in the third embodiment of the present invention; FIG. 16 is an illustration showing the method for constructing a reference minimum voltage determining table in the third embodiment of the present invention; FIG. 17 is a flow chart showing the reference current value correcting operation of the current condition computing section in the fourth embodiment of the present invention; FIG. 18 is an illustration showing the interpolating operation executed in the fourth embodiment; FIG. 19 is an illustration showing the relationship between V dc1 and V dc2 in FIG. 18; FIG. 20 is a circuit diagram showing an equivalent circuit per phase of a PM motor; FIG. 21 is a vector diagram of a PM motor; and FIG. 22 is an illustration showing the relationship between motor rotational speed and torque when field weakening control is performed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention are described below by referring to the accompanying drawings. (a) System constitution FIG. 1 shows a system environment suitable for executing each embodiment of the present invention. The system shown in FIG. 1 is an electric vehicle drive system and its controller. In the system shown in FIG. 1, the electric discharge output of a battery 10 is converted from DC to 3-phase AC by an inverter 12 and fed to a PM motor 14. The power conversion operation in the inverter 12 is vector-controlled by a current condition computing section 15 and a current control section 18. The current condition computing section 16 determines a field reference current I d * and reference torque current I q * in accordance with a reference torque T* having a value corresponding to an amount of depression of an accelerator or brake pedal by a driver given from a supervisor control section (vehicle controller, regeneration controller, or the like), and feeds the reference currents to the current control section 18. The current control section 18 inputs not only these reference currents I d * and I q * but also the angular position  of the rotor of the motor 14 detected by a motor rotational-position sensor 20 affixed to the motor 14. The current control section 18 performs the rotational transform of the current vector coordinate in correspondence to the position , in order to realize the alternation of current, and simultaneously generates U, V, and W phase reference currents I U *, I V *, and I W * in accordance with the reference currents I d * and I q * to feed them to the inverter 12. The inverter 12 generates, for example, PWM (pulse width modulation) signals in accordance with the fed reference currents I U *, I V * and I W *. The inverter 12 comprises, for example, a plurality of switching devices such as an IGBTs (insulated gate bipolar transistors). These switching devices are switched by the PWM signals generated by conversion so that currents I U , I V , and I W with values corresponding to the reference currents I U *, I V *, and I W * flow through the U, V, and W phases of the motor 14. When this type of switching is performed, the field current I d * and torque current I q * having values which correspond to the reference currents I d * and I q * flow through the U, V, and W phases of the motor 14 and these currents generate the alternating magnetic field. The torque T outputted from the motor 14 as the result of the above current control comes to a value corresponding to the reference torque T*. The current condition computing section 16 normally controls the reference field current I d * to a constant value, for example, 0. Therefore, the field flux in the motor 14 is kept constant. The current condition computing section 16 changes the reference torque current I q * in accordance with a change of the reference torque T*. Thus, since the time constant for responding to the torque current component I q * is small, it is possible to rapidly change the output torque of the motor 14. Moreover, the current condition computing section 16 executes the field weakening control. Therefore, the current condition computing section 16 detects the rotational speed N of the motor 14 with the motor rotational-position sensor 20. In the present invention, the value of the field weakening reference current I d * is changed in accordance with the battery voltage V dc . Thus, it is possible to favorably execute the field weakening control in accordance with a change of the battery voltage V dc . In the system shown in FIG. 1, a voltage sensor 22 is affixed to the battery 10 to realize the above control, and the voltage V dc between both terminals of the battery 10 is detected by the voltage sensor 22. It is also possible to estimate the battery voltage V dc in accordance with an SOC of the battery 10 or a vehicle state such as a road load. In FIG. 1, a member 24 set to the battery 10 is an SOC sensor for detecting an SOC of the battery 10. The present invention is not restricted to the system constitution of FIG. 1. (b) First embodiment FIG. 2 shows the flow of operation of the current condition computing section 16 in the first embodiment of the present invention. The current condition computing section 16 receives the reference torque T* from a not-illustrated supervisor controller and the motor rotational speed N from the motor rotational-position sensor 20, respectively (step 100). The current condition computing section 16 compares the inputted motor rotational speed N with the base rotational speed N B of the PM motor 14 (102). When the rotational speed N is lower than the base rotational speed N B , the current condition computing section 16 computes the torque current I q by substituting " ω=2πN ", "I d =0" and " T=T* " for the equation (1) (step 104) and outputs the obtained torque current I q as the reference torque current I q * (step 106). Because the main magnetic flux E 0 is already known, the above computation is possible. In this case, the current condition computing section 16 outputs 0 or a constant value as the field reference current I d *. Normal field control is performed in the above manner. When the rotational speed N is equal to or higher than the base rotational speed N B , the current condition computing section 16 performs the field weakening control. This embodiment is characterized by the improved field weakening control. For the field weakening control, the current condition computing section 16 first computes the maximum motor-applied voltage V MAX in accordance with the battery voltage V dc detected by the voltage sensor 22 (108). In more detail, the current condition computing section 16 converts the present battery voltage V dc from an input-side value to an output-side value, of the inverter 12. An obtained converted value is used as the maximum motor-applied voltage V MAX . In this case, the current condition computing section 16 assumes the PWM modulation factor of the inverter 12 to be 100%. Therefore, the maximum motor-applied voltage V MAX comes to a value representing the highest inverter output voltage obtained from the present battery voltage V dc . Particularly, when a triangular wave is used as the carrier wave of a PWM signal in the inverter 12, the current control section 18 gives the reference currents I U *, I V *, and I W * to the inverter 12 as sine-wave signals, and the inverter 12 generates the PWM signals by comparing these sine-wave signals with the carrier wave, respectively, the arithmetic equation evaluated in the step 108 is shown as follows: V MAX = 3 1/2 V B /(2*2 1/2 ) Needless to say, this is just one example of PWM signal generating methods. In the present invention, it is also possible to use a wave other than the triangular wave as the carrier wave. In this case, although the above equation (5) cannot directly be used, a person skilled in the art can easily modify the equation (5) based on the disclosure of this application. The current condition computing section 16 computes the field weakening current I d and the torque current I q by substituting " ω=2πN ", " T=T* " and " V=V MAX " for the equations (1) to (3) (110). That is, because L d , L q , R, and E o are already known, it is possible to solve these equations for I d and I q by applying ω, T and V to these equations (1) to (3). Because the term of I d I q is included in the equation (1), the operation is complicated. To avoid this problem, it is recommended to prepare and store a map for obtaining I d and I q corresponding to N, T and V by referring to N, T and V as keys. Thus, it is possible to execute the step 110 simply by referring to the map. It is also possible to use the equation (4) instead of the equation (2). Then, the current condition computing section 16 judges whether a mode command given from an operator through a supervisor controller indicates selection of an output mode or an efficiency mode (112). When the section 16 judges that the mode command indicates "output mode", it executes the step 118 by skipping the steps 114 and 116. When the section 16 judges that the mode command indicates "efficiency mode", it executes the step 118, after executing the steps 114 and 116. In the step 114, the current condition computing section 16 judges whether an SOC detected by the SOC sensor 24 is the predetermined value SOC 0 or less, that is, whether the SOC has degraded. When the section 16 judges that the SOC has not degraded, it skips the step 116. When the section 16 judges that the SOC has degraded, it limits the value of the field weakening current I d computed in the step 110 to a predetermined value I d0 (116). The field current I d and torque current I q thus obtained are outputted to the inverter 12 as the field weakening reference current I d * and the reference torque current I q * (118). The field weakening control in this embodiment is performed in the above manner. For this embodiment, as described above, the actual battery voltage V dc is considered to determine the field weakening reference current I d * and the reference torque current I q *. For example, when the battery voltage V dc fluctuates due to a change of the SOC of the battery 10 or depending on a load of the PM motor 14, the maximum motor-applied voltage V MAX changes in correspondence with the fluctuation of the battery voltage V dc . Thereby, the field weakening reference current I d * and the reference torque current I q * computed in accordance with the maximum motor-applied voltage V MAX are also changed. This embodiment realizes the optimum field weakening control for preventing too-large or too-small a field weakening current I d from flowing, due to the above control. As shown in FIG. 3, by increasing the absolute value of the field weakening current I d , the terminal voltage V is lowered and also the efficiency η decreases. That is, when the field weakening current I d increases, the primary current I=(I d * 2 +I q * 2 ) 1/2 of the PM motor 14 increases, the copper loss increases due to the increase of the current I, and resultingly the efficiency η decreases. Moreover, it is found from FIG. 3 that the efficiency η is maximized at the terminal voltage V equal to or lower than the maximum motor-applied voltage V MAX when the field weakening current I d at V=V MAX is used as the field weakening reference current I d *. Therefore, when the maximum motor-applied voltage V MAX is computed in accordance with the battery voltage V dc , and the reference field current I d * is determined by using the V MAX , deterioration of the efficiency η due to excessive flow of the field weakening current I d or output deficiency due to insufficient field weakening current I d does not occur. In more detail, the reason why the reference field current I d * in this embodiment should be set in response to the SOC and therefore to the maximum motor-applied voltage V MAX is presented in the relationship shown in FIG. 3A. As is shown in this figure, the terminal voltage V decreases as the absolute value of the field weakening current I d increases, and it also decreases as the revolution speed N of the motor 14 decreases. The maximum motor-applied voltage V MAX decreases as the SOC of the battery 10 decreases. As has been described hereinbefore, since the situation V>V MAX must not occur, the field weakening current I d must be controlled so that the terminal voltage V is equal to or less than V MAX . The filed weakening current I d becomes I dmin when V=V MAX . I dmin is determined depending on the revolution speed N and the SOC (or V MAX ), as shown in FIG. 3A. In this embodiment, the reference field current I d * is set in response to the maximum motor-applied voltage V MAX , on the basis of the above described relationship. In other words, the reference field current I d * is controlled so that it is set larger when the SOC or the maximum motor-applied voltage V MAX decreases, because the absolute value of I dmin becomes large. Thus, by using such an I d * control adaptive to the SOC, the minimum weakening field current I dmin is obtained, which results in the improved efficiency. In this embodiment, operations of the current condition computing section 16 are changed in accordance with a mode command. Therefore, driving stressing the output power or driving stressing the efficiency can be carried out, depending on the preference of a vehicle driver. A vehicle driver can limit the maximum value of the absolute value of the field weakening reference current I d * to -I d0 by giving, for example, a mode command for selecting the efficiency mode to the current condition computing section 16. That is, when the battery voltage V dc lowers in accordance with the degradation of an SOC, the maximum motor-applied voltage V MAX computed in the step 108 lowers and, as shown in FIG. 4, the absolute value of the field weakening current I d increases in accordance with the decrease of the voltage V MAX . If the field weakening current I d with an increased absolute value is directly outputted as the field weakening reference current I d *, the efficiency decreases because a current component (I d ) that is not contributing to generation of torque, increases. In this embodiment, the value of the field weakening reference current I d * is limited to the predetermined value I d0 in accordance with the degradation of an SOC when the efficiency mode is selected. Therefore, a vehicle driver, particularly a driver stressing the efficiency, can execute driving with the emphasis on efficiency, in accordance with his preference. A vehicle driver can obtain a PM-motor output not limited by I d0 in the high rotation region by issuing a mode command for selecting the output mode to the current condition computing section 16 independently of degradation of an SOC. That is, when the above field current limitation is executed, the output range of torque T is narrowed to the low-torque side and as a result output limitation occurs since the motor output is determined by TN. Therefore, this embodiment uses the output mode for omitting the steps 116 and 118 so that the output torque T can be secured. Moreover, the output limitation resulting from by the field current limitation does not cause the problem seen in, for example, Japanese Patent Laid-Open No. Hei 5 - 38003. In this official gazette, the output torque T of a PM motor is limited in accordance with degradation of an SOC as shown in FIG. 6. In FIG. 6, the maximum output torque in the high rotation region is changed from an isopower line with a comparatively high power (line where the PM-motor output power obtained as a product of the output torque T and rotational speed N is constant) to an isopower line with less power in accordance with the degradation of an SOC. In the case of this output limitation, the output torque T is not limited even in the high rotation region as long as the reference torque T* is low. In this case, however, a loss occurs because field weakening control is performed. Moreover, output is limited even in the medium rotation region in which a high efficiency is to be obtained. In this embodiment, the output limitation due to the field current limitation executed in accordance with degradation of an SOC functions even in the high-rotation low-torque region as shown in FIG. 5. Therefore, the efficiency is prevented from being lowered in spite of the increase of the absolute value of the field current I d caused by degradation of the SOC, and moreover the accelerating performance from standstill is secured and the drive feeling is favorably secured. Moreover, because the output limitation is not performed in the medium rotation region, a high efficiency is also obtained from this point of view. In the above description, the only case which is shown is one in which the current vector I of the motor 14 is decomposed into the field current I d and the torque current I q to perform vector control. However, it is also possible to perform vector control by decomposing the current I into the absolute value |I| and the phase argI of the motor current I. This is also applicable to the other embodiments. (c) Principle of second to fourth embodiments In the first embodiment, the field weakening reference current I d * is determined in accordance with the battery voltage V dc . However, the battery voltage V dc is also caused to fluctuate by applying a load to the PM motor 14 which is a driving object. That is, even if the vector of the motor current I is controlled in accordance with the battery voltage V dc detected at the time t=0, the battery voltage V dc is caused to fluctuate due to the load of the PM motor 14 and frequently converges to a value V s lower than the value at the time t=0 as shown in FIG. 7. Therefore, when control is performed in accordance with the battery voltage V dc at the time t=0 so that the best efficiency is obtained, necessary current may not be obtained from the battery 10 and thereby output may be insufficient for the reference torque T*. The second to fourth embodiments described below settle the fluctuation of the battery voltage V dc due to the load of the PM motor 14. That is, the embodiments make it possible to operate the PM motor 14 under conditions for maximizing the efficiency by using the approximation set out below as a main principle. FIGs. 8 and 9 show the field current I d and torque current I q for maximizing the efficiency of the PM motor 14, that is, the optimum field current I dp and the optimum torque current I qp . Particularly, FIG. 9 shows a case in which a salient-pole motor is used as the PM motor 14. As shown in FIGs. 8 and 9, the optimum field current I dp and the optimum torque current component I qp , when using a salient-pole motor as the PM motor 14, depend on the motor rotational speed N, reference torque T*, and battery voltage V dc . The field current I d is negative. Moreover, attention should be paid to the fact that for simplification only, T* is drawn up to 100% in the case of N=0 and N=N 1 (>0) and up to 40% in the case of N=N 2 (>N 1 ). As shown in FIG. 8, when the motor rotational speed N equals 0, the value of the optimum field current I dp is constant, independent of the battery voltage V dc but it varies in accordance with the value of the reference torque T*. When the motor rotational speed N increases and reaches N 1 , and moreover reaches N 2 , the value of the optimum field current I dp changes in a region where the battery voltage V dc is low. That is, the characteristic of the optimum field current I dp to the battery voltage V dc is inclined. This is because the counter electromotive force of the motor 14 exceeds the battery voltage V dc if the normal field control is performed and therefore the above described field weakening control is executed to prevent the force from exceeding the voltage. Moreover, as shown in FIG. 9, when the motor rotational speed N is 0, the optimum torque current component I qp is constant independently of the battery voltage V dc . However, when the motor rotational speed N reaches N 1 , and moreover reaches N 2 , the component I qp is inclined in a region where the battery voltage V dc is low. This is because the motor 14 is a salient-pole motor and thereby generates reluctance torque, that is, torque depending on the field current I d {see the second term of the right side of the equation (1)} and therefore it is necessary to decrease the torque current I q when increasing the absolute value of the field current I d in a region where the battery voltage V dc is low in order to maintain an amplitude of the primary current I and to perform field weakening control. The relationships shown in FIGs. 8 and 9, that is, the relationship between the optimum field current I dp , optimum torque current I qp , and battery voltage V dc , can be approximated as shown in FIGs. 10 and 11. First, the optimum field current I dp , as shown in FIG. 10, can be approximated by the straight line of inclination k d (I d1 * in FIG. 10) in a region where the battery voltage V dc is low and by the value of the optimum field current I dp (I d2 * in FIG. 10) when the motor rotational speed N is 0 in a region where the battery voltage V dc is high. Then, the optimum torque current I qp , as shown in FIG. 11, can be approximated by the straight line of inclination k q (I q1 * in FIG. 11) in a region where the battery voltage V dc is low and by the value of the torque current component I q (I q2 * in FIG. 11) when the motor rotational speed N is 0 in a region where the battery voltage V dc is high. The inclination vector (k d , k q ) has a value corresponding to the reference torque T* and motor rotational speed N. These approximate equations can be expressed as follows: I dp = I d1 * for ΔV dc < 0 = I d2 * for ΔV dc ≥ 0 I qp = I q1 * for ΔV dc < 0 = I q2 * for ΔV dc ≥ 0 I d1 * = I dmin + k d *ΔV dc I q1 * = I qmin + k d *ΔV dc ΔV dc = V dc - V dcmin V dcmin : A minimum value of V dc which can be outputted from the battery 10 (Reference minimum voltage) I dmin : I dp for V dc = V dcmin I qmin : I qp for V dc = V dcmin (d) Second embodiment FIG. 12 shows an overall flow of operation of the current condition computing section 16 in the second embodiment of the present invention. This flow is executed at a predetermined frequency. In this embodiment, the current condition computing section 16 first receives the reference torque T* from the supervisor controller (200) and detects the motor rotational speed N with the motor rotational-position sensor 20 (202). Thereafter, the current condition computing section 16 sets the minimum value of the battery voltage V dc which can be outputted from the battery 10 to the reference minimum voltage V dcmin (204). The current condition computing section 16 obtains the values of the optimum field current I dp and the optimum torque current I qp when the battery voltage V dc equals the reference minimum voltage V dcmin , that is, the intercept vector (I dmin , I qmin ) of the equation (6) in accordance with the set reference minimum voltage V dcmin . In this case, the current condition computing section 16 refers to a previously prepared table 300. This table makes the motor rotational speed N and the reference torque T* correspond to the intercept (I dmin , I qmin ). This table can be prepared in accordance with the relationships shown in FIGs. 8 and 9. Thereafter, the current condition computing section 16 judges whether the reference torque T* and the motor rotational speed N change by as much as a predetermined value or more (208). That is, the section 16 computes a change ΔT* in reference torque T* and a change ΔN in motor rotational speed N between the reference torque T* and the motor rotational speed N respectively, computes this time and the previous time, and judges whether the obtained changes ΔT* and ΔN exceed the predetermined values ΔT 0 and ΔN 0 respectively. As the result of the judgment, when either of the inequalities ΔT*>ΔT 0 and ΔN>ΔN 0 is effected, the current condition computing section 16 commences step 210 by assuming that the load of the motor 14 increases, while it commences the step 212 for cases other than the above. In the step 210, the current condition computing section 16 outputs the intercept (I dmin , I qmin ) computed in the step 206 to the current control section 18 as the reference current (I d *, I q *). Thereby, the vector of the current I of the motor 14 is controlled in accordance with the intercept (I dmin , I qmin ), and in turn, the reference minimum voltage V dcmin of the battery voltage V dc . Under the above state, the efficiency of the motor 14 is not maximized. However, the output power of the motor 14 obtained by using the intercept (I dmin , I qmin ) as the reference current (I d *, I q *) comes to a value equivalent to the value obtained by multiplying the reference torque T* by the motor rotational speed N. Therefore, even if the battery voltage V dc fluctuates due to the load of the motor 14 as shown in FIG. 7, at least the requested output power of the motor 14 can be secured. Thereafter, the current condition computing section 16 waits until the battery voltage V dc is converged (214, 216). Once the battery voltage V dc is sufficiently converged, the current condition computing section 16 corrects the values of the reference current (I d *, I q *) in accordance with a converged battery voltage V s (212) and feeds the corrected reference current (I d *, I q *) to the current control section 18 (212). Thereby, the motor 14 is brought into a state in which the maximum efficiency is obtained. FIG. 13 shows the details of the step 212, that is, the details of correction of the reference current (I d * , I q *) in this embodiment. In this embodiment, the current condition computing section 16 first computes the inclination (k d , k q ) to correct the reference current (I d *, I q *) (220). In this case, the current condition computing section 16 uses a table 302 for making the reference torque T* and the motor rotational speed N correspond to the inclination (k d , k q ). This table can be prepared in accordance with the relationships shown in FIGs. 8 and 9. The current condition computing section 16 substitutes the obtained inclination (k d , k q ) and the intercept (I dmin , I qmin ) for the equation (6) and also substitutes the converged battery voltage V s for the V dc of the equation (6) to compute an approximate current vector (I d * 1 , I q * 1 ) (222). The approximate current (I d * 1 , I q * 1 ) thus obtained can be used as the reference current (I d *, I q *) when the battery voltage V dc is low. Thereafter, the current condition computing section 16 reads the vector of the motor current I for realizing the maximum efficiency when the motor rotational speed N is 0, that is, the approximate current vector (I d * 2 , I q * 2 ) shown in FIGs. 10 and 11, from a built-in memory (224). The approximate current (I d * 2 , I q * 2 ) read in at step 224 can be used as the reference current (I d *, I q *) in a region where the battery voltage V dc is high as shown in FIGs. 10 and 11. In the steps 226 and 228, the current condition computing section 16 compares the approximate current (I d * 1 , I q * 1 ) obtained in the step 222 with the approximate current (I d * 2 , I q * 2 ) read in the step 224. In step 226, the judgement relating to the field current is made. In the case of "I d * 1 < I d * 2 ", it can be considered that the present battery voltage or the converged battery voltage V s is included in the left region in FIG. 10. That is, it can be considered that the converged battery voltage V s is in a region where the value of the optimum field current I dp depends on the value of the battery voltage V dc . Therefore, the current condition computing section 16 sets the approximate field current I d * 1 obtained in the step 222 to the reference field current I d * to be outputted in the step 218 (230). In the case of "I d * 1 ≥ I d * 2 ", however, it can be considered that the present battery voltage V s is included in the right region in FIG. 10. That is, it can be considered that the converged battery voltage V s is in the region where the value of the optimum field current I dp does not depend on the value of the battery voltage V dc . Therefore, the current condition computing section 16 sets the approximate field current I d * 2 read in the step 224 to the reference field current I d * to be outputted in the step 218 (232). Moreover, the current condition computing section 16 judges in the step 228 whether I q * 1 < I q * 2 or not. When the current condition computing section 16 judges that "I q * 1 < I q * 2 ", it can be considered that the value of the optimum torque current component I qp is included in the left region (the region where the value of the optimum torque current I qp depends on the value of the battery voltage V dc ) in FIG. 11. Therefore, the current condition computing section 16 sets the approximate torque current I q * 1 obtained in the step 222 to the reference torque current I q * to be outputted in the step 218 (234). When the current condition computing section 16 judges that "I q * 1 ≥ I q * 2 ", however, it can be considered that the battery voltage V s is included in the right region in FIG. 11, that is, the region where the value of the optimum torque current component I depends on the value of the battery voltage V dc . In this case, the current condition computing section 16 sets the approximate torque current I q * 2 read in the step 224 to the reference torque current I q * to be outputted in the step 218 (236). Thus, the reference current (I d *, I q *) in this embodiment is corrected. Therefore, in this embodiment, because the reference torque T* and motor rotational speed N are inputted and detected and then the vector of the motor current I is initially controlled in accordance with the reference minimum voltage V dcmin , the problem does not occur that necessary output power cannot be obtained from the motor 14 even when the battery voltage V dc fluctuates due to the load of the motor 14. Moreover, in this embodiment, it is possible to quickly control the efficiency of the motor 14 to the maximum value because the above initial control is performed and the battery voltage V dc is converged and thereafter the vector of the motor current I is controlled in accordance with the converged battery voltage V s . Furthermore, the maximum efficiency control of the motor 14 is performed by approximating the relation between the optimum current (I dp , I qp ) for maximizing the efficiency of the motor 14 and the battery voltage V dc with the linear approximation equation in the equation (6) and correcting the initial reference current {= intercept (I dmin , I qmin )} with the linear approximation equation. Therefore, it is possible to realize the maximum efficiency control of the motor 14 without storing a lot of data showing the relationship between the reference torque value T*, motor rotational speed N, and battery voltage V dc . That is, it is only necessary to store the inclination (k d , k q ) of the linear approximation equation in a region where the battery voltage V dc is low and the values of the approximate current (I d * 2 , I q * 2 ) when the motor rotational speed N is 0 in a region where the battery voltage V dc is high. Therefore, it is not necessary to use an expensive CPU having a large memory capacity in order to perform the maximum efficiency control of the motor 14. FIG. 14 shows the control timing in this embodiment. As shown in FIG. 14, the reference torque T* and motor rotational speed N are inputted and detected every predetermined sampling period. When the reference torque T* is inputted and the motor rotational speed N is also inputted, the processing ("initial computation" in FIG. 14) in the step 206 is performed. When the intercept (I dmin , I qmin ) is obtained, the vector of the motor current I is controlled in accordance with the intercept. After this control is executed, the battery voltage V dc changes due to the load of the motor 14. When the fluctuation of the value of the battery voltage V dc gradually decreases and, for example, dV dc /dt becomes smaller than a predetermined microvalue δ (see FIG. 7), the current condition computing section 16 judges that the battery voltage V dc has converged to V s and executes the processing ("corrective computation" in FIG. 14) in the step 212. After the resulting reference current (I d *, I q *) is outputted, the efficiency of the motor 14 is maximized. In this case, symbol T in FIG. 14 represents an actual value of the output torque (actual torque) of the motor 14, which becomes equal to the reference torque value T* when the initial computation is completed and (I dmin , I qmin ) is outputted. In FIG. 14, the first-time initial computation is executed due to a change of the reference torque T* and the second-time initial computation is executed due to a change of the motor rotational speed N. It is also possible to use a non-salient-pole motor as the motor 14. In this case, the optimum torque current I qp does not decrease in the low rotation region shown in FIG. 9. Moreover, the approximation equation used in the step 212 is not restricted to a linear equation but it is possible to use, for example, a polynomial equation. That is, it is possible to use any equation as an approximation equation in the second embodiment as long as the equation passes through the intercept (I dmin , I qmin ) and approximates the relationship between the optimum field current I dp and the optimum torque current I qp to the battery voltage V dc . These points also apply to the following embodiments. (e) Third embodiment FIG. 15 shows a flow of control of the current condition computing section 16, particularly a flow of the processing in step 204. In this embodiment, the current condition computing section 16 detects an SOC of the battery 10 with the SOC sensor 24 (238) and also detects the battery voltage V dc with the voltage sensor 22 (240). The current condition computing section 16 computes the value of an output reference power P* to the motor 14 by multiplying together the reference torque T* and the motor rotational speed N inputted and detected in the steps 200 and 202, respectively (242). Then, the current condition computing section 16 estimates the converged battery voltage V s in accordance with the obtained output reference power P* and the detected SOC (244). In this case, the current condition computing section 16 refers to a table 204 showing the relationship between the output P*, SOC, and battery voltage V s . That is, the converged battery voltage V s depends on the output P* and SOC as shown in FIG. 16 and this relationship is stored in the current condition computing section 16 as a table 304. The current condition computing section 16 sets the reference minimum voltage V dcmin in accordance with the estimated converged battery voltage V s (246). That is, it sets V dcmin by subtracting a predetermined value ΔV from V s . When the motor current (I d , I q ) is controlled in accordance with the reference minimum voltage V dcmin thus set (210), it is possible to secure the output of the motor 14 even though the battery voltage V dc fluctuates due to a load as described above. Moreover, when the reference minimum voltage V dcmin is set by considering an SOC of the battery 10 as is done in this embodiment, it is not necessary to excessively decrease the reference minimum voltage V dcmin and it is possible to control the efficiency of the motor 14, before convergence of the battery voltage after execution of the step 110, to a reasonably high value, although the value is not maximum. (f) Fourth embodiment FIG. 10 shows a flow of the operation of the current condition computing section 16 in the fourth embodiment of the present invention, particularly a flow of the reference current correcting operation in step 212. In this embodiment, the current condition computing section 16 computes the reference current (I d *, I q *) by using a table 306. The table 306 includes a plurality of unit tables for making the optimum current (I dp , I qp ) correspond to the reference torque T* and motor rotational speed N. Each of these unit tables is made to correspond to each digitized value of the battery voltage Vdc. To compute the reference current (I d *, I q *) by using the table 306, the current condition computing section 16 first selects V dc1 and V dc2 which are the closest to the converged battery voltage V s out of the battery voltages V dc included in the table 306 (248). The current condition computing section 16 computes the reference currents (I d * 1 , I q * 1 ,) and (I d * 2 and I q * 2 ) corresponding to the values V dc1 and V dc2 respectively by using not only the reference torque T* and the motor rotational speed N but also V dc1 and V dc2 as keys, and referring to the table 306 (250). The current condition computing section 16 computes the reference current (I d *, I q *) to be outputted in the step 218 through the interpolation of the obtained reference currents (I d * 1 , I q * 1 ) and (I d * 2 , I q * 2 ) (252). Therefore, in this embodiment, the reference current (I d *, I q *) with the value shown in FIG. 18 is outputted. However, the inclination of (I d , I q ) is that in the voltage region shown in FIG. 19. (g) Supplement As described above, the present invention makes it possible to obtain the optimum field weakening current I d independently of the fluctuation of the SOC of a battery or that of the load of a PM motor because the field weakening current I d is controlled in accordance with a change of the battery voltage V dc . Therefore, it is possible to maintain the predetermined efficiency η without causing excessive field weakening current to flow when the battery voltage V dc is comparatively high and to obtain the current I required to obtain the necessary torque T when the battery voltage V dc is comparatively low. Moreover, when the allowable power-source voltage V B is computed in accordance with the battery voltage V dc and the PWM modulation rate of an inverter, the above advantage can preferably be obtained in controlling the terminal voltage V of a PM motor through the PWM control. Furthermore, by limiting the field weakening current I d in accordance with the SOC of a battery when the field weakening current I d is computed in accordance with the battery voltage V dc , the efficiency η can be prevented from being lowered because a decrease of the battery voltage V dc due to degradation of the SOC does not cause the field weakening current I d to increase. Also in this case, an accelerating performance from standstill is secured because the torque T is secured in a low-rotation high-torque region. Therefore, by using the PM motor as the driving mechanism of an electric vehicle, it is possible to secure a preferable drive feeling. Furthermore, when limitation of the field weakening current I d is executed or stopped in accordance with, for example, a command from a user, the user can select drive in consideration of efficiency or drive in consideration of output. Thus, it is possible to flexibly satisfy the user's preference. Furthermore, in the case where the vector of the motor current I is initially controlled on the assumption that the battery voltage V dc is the reference minimum voltage V dcmin and at the time of the convergence of the battery voltage V dc after this vector control, the vector of the motor current I is controlled in accordance with the converged battery voltage V s , the motor output power can be prevented from decreasing even if the battery voltage fluctuates due to load after vector control. Furthermore, because the motor efficiency η can be controlled to the maximum value after the battery voltage V dc is converged, it is possible to obtain a driving and controlling apparatus having a high energy efficiency and suitable for mounting on an electric vehicle or the like. Furthermore, when the reference minimum voltage V dcmin is set in accordance with the SOC of a battery and the voltage V dc , the reference minimum voltage V dcmin can be prevented from being excessively lowered, and a comparatively high motor efficiency η can be realized, even before the battery voltage V dc is converged. Furthermore, when the relationship between the battery voltage V dc and the optimum current (I dp , I qp ) is approximated by a mathematical equation in a region where the optimum current for maximizing the motor efficiency η depends on the battery voltage V dc to store the coefficient such as the inclination (k d , k q ) of the approximation equation or the optimum current (I d * 1 , I q * 1 ) are stored in a region where the optimum current (I dp , I qp ) does not depend on the battery voltage Vdc, it is possible to execute vector control after the battery voltage V dc is converged by using these stored pieces of information. Therefore, it is not necessary to store a complicated relationship between the reference torque value T*, motor rotational speed N, and battery voltage V dc and an apparatus which can be realized at a low cost is obtained. Furthermore, when the above approximation equation is used as a linear equation and the inclination (k d , k q ) or intercept of the linear equation are stored as the coefficient, it is possible to make the advantage of decreasing the stored information content more remarkable. An Intercept can use the initial reference current (I dmin , I qmin ). When a rotational speed (N) of a synchronous motor (14) using permanent magnets as its field system is equal to or higher than a base rotational speed (N B ), a current condition computing section (16) computes a field weakening reference current (I d *) in accordance with a battery voltage (V dc ), reference torque (T*), and the rotational speed (N). An inverter (12) performs the field weakening control of the synchronous motor (14) in accordance with the field weakening reference current (I d *) sent from the current condition computing section (16) to prevent the deterioration of efficiency and the deficiency of output due to excessive or insufficient field weakening current (I d ). The current condition computing section (16) determines an optimum reference current (I d *, I q *) by using a converged battery voltage (V s ) when the battery voltage (V dc ) is converged after initial control.

Claims: A driving and controlling apparatus for use in a drive unit having a synchronous motor (14) using permanent magnets as its field system which can be excited by a field flux (E 0 ) generated by the permanent magnets and a field flux generated by a field current component (I d ) in a primary current (I); a battery (10) as a source of power; and power conversion means (12) for converting power discharged from the battery (10) into alternating current and feeding the alternating current as the primary current (I) to the synchronous motor (14); said apparatus comprising: current control means (18) for controlling the primary current (I) of the synchronous motor (14) in accordance with a reference current (I d *, I q *); and current condition determination means (16) for carrying out a field weakening being characterized in that said current condition determination means (16) is provided for minimizing the reference current (I d *, I q *) in accordance with a voltage (V dc ) of the battery (10) under predetermined conditions and at least partially cancelling the field flux (E 0 ) generated by the permanent magnet with the field flux generated by the field current component (I d ) so that a counter electromotive force (ωE 0 ) of the synchronous motor (14) does not exceed the voltage (V dc ) of the battery (10) and so that the field flux generated by the field current component (I d ) varies as the voltage of the battery (10) varies. The driving and controlling apparatus according to Claim 1 wherein the current condition determination means (16) judges whether or not the counter electromotive force (ωE 0 ) of the synchronous motor (14) exceeds the voltage (V dc ) of the battery (10) in accordance with an output state (T, N) of the synchronous motor (14), determines the reference current (I d *, I q *) so that the field flux generated by the field current component (I d ) is fixed to 0 or a constant value, when it has been judged that the counter electromotive force (ωE 0 ) does not exceed the voltage (V dc ) of the battery (10), and determines the reference current (I d *, I q *) so that the counter electromotive force (ωE 0 ) of the synchronous motor (14) does not exceed the voltage (V dc ) of the battery (10) and so that the field flux generated by the field current component (I d ) has the intensity corresponding to the voltage (V dc ) of the battery (10), when it has been judged that the counter electromotive force (ωE 0 ) exceeds the voltage (V dc ) of the battery (10). The driving and controlling apparatus according to Claim 2 wherein the output state of the synchronous motor (14) used to judge whether or not the counter electromotive force (ωE 0 ) of the synchronous motor (14) exceeds the voltage (V dc ) of the battery (10) is a rotational speed (N) of the synchronous motor (14), and the current condition determination means (16) judges that the counter electromotive force (ωE 0 ) of the synchronous motor (14) exceeds the voltage (V dc ) of the battery (10), when the rotational speed (N) of the synchronous motor (14) is higher than a predetermined value (N B ), and judges that the counter electromotive force (ωE 0 ) does not exceed the voltage (V dc ) of the battery (10), when the rotational speed (N) is not higher than the predetermined value (N B ). The driving and controlling apparatus according to any one of Claims 1 to 3 wherein the reference current includes the reference field current (I d *) showing a control target of the field current component (I d ) of the primary current (I) and the reference torque current (I q *) showing a control target of a torque current component (I q ) of the primary current (I); and the current condition determination means (16) determines the reference field current (I d *) in accordance with at least the voltage (V dc ) of the battery (10) so that the counter electromotive force (ωE 0 ) of the synchronous motor (14) does not exceed the voltage (V dc ) of the battery (10) and so that the field flux generated by the field current component (I d ) has an intensity corresponding to the voltage (V dc ) of the battery (10), and determines the reference torque current (I q *) in accordance with at least the reference torque (T*) so that torque corresponding to a reference torque (T*) is outputted from the synchronous motor (14). The driving and controlling apparatus according to Claim 4 wherein the current condition determination means (16) further refers to the reference torque (T*) in order to determine the reference field current (I d *), and further refers to the voltage (V dc ) of the battery (10) in order to determine the reference torque current (I q *). The driving and controlling apparatus according to Claim 5 wherein the current condition determination means (16) converts the voltage (V dc ) of the battery (10) into a value (V MAX ) comparable with a terminal voltage (V) of the synchronous motor (14), and derives information regarding the field current component (I d ) and the torque current component (I q ) from an equation or a map showing a relationship among the terminal voltage (V), the field current component (I d ), the torque current component (I q ) and the torque (T) of the synchronous motor (14) when the converted voltage (V MAX ) of the battery (10) is used as the terminal voltage (V) and the reference torque (T*) is used as the torque (T), thereby determining the reference field current (I d *) and the reference torque current (I q *). The driving and controlling apparatus according to Claim 6 wherein the current condition determination means (16) converts the voltage (V dc ) of the battery (10) into the value (V MAX ) comparable with the terminal voltage (V) on the assumption that a power conversion rate of the power conversion means (12) is maximum. The driving and controlling apparatus according to Claim 6 or 7 wherein the current condition determination means (16) limits the value of the field current component (I d ), when the remaining capacity (SOC) of the battery (10) is not more than a predetermined value (SOC 0 ), thereby decreasing the cancelled quantity of the field flux (E 0 ) generated by the permanent magnet which is cancelled with the field flux generated by the field current component (I d ). The driving and controlling apparatus according to Claim 8 wherein the current condition determination means (16) prohibits the limitation of the field current component (I d ) in accordance with a mode command showing an efficiency mode. The driving and controlling apparatus according to any one of Claims 1 to 5 wherein the current condition determination means (16) determines an initial reference current (I dmin , I qmin ) in accordance with the reference torque (T*), the output state (N) and a reference minimum voltage (V dcmin ) on the assumption that the voltage (V dc ) of the battery (10) is the reference minimum voltage (V dcmin ), when either of the reference torque (T*) or the output state (N) of the synchronous motor (14) change by as much as a predetermined value or more, gives the determined initial reference current (I dmin , I qmin ) as the reference current (I d *, I q *) to the current control means (18) and, when the voltage (V dc ) of the battery (10) is converged to a constant value after the primary current (I) of the synchronous motor (14) has been controlled in accordance with the initial reference current (I dmin , I qmin ), determines the reference current (I d *, I q *) in accordance with the converged voltage (V s ) of the battery (10). The driving and controlling apparatus according to Claim 10 wherein the current condition determination means (16) determines the reference minimum voltage (V dcmin ) in accordance with the remaining capacity (SOC) and voltage (V dc ) of the battery (10). The driving and controlling apparatus according to Claim 10 or 11 wherein an optimum current (I dp , I qp ) depends on the voltage (V dc ) of the battery (10) when the output state (T, N) of the synchronous motor (14) is in a predetermined region and does not depend on the voltage (V dc ) of the battery (10) when the output state (T, N) of the synchronous motor (14) is not in the predetermined region, the optimum current (I dp , I qp ) representative of the reference current(I d *, I q *) where the reference torque (T*) is realized as the torque (T) of the synchronous motor (14) and the efficiency of the synchronous motor (14) substantially becomes a maximum efficiency; the current condition determination means (16), in a region where the values of the optimum current (I dp , I qp ) depends on the voltage (V dc 0 of the battaery (10), computes first optimum reference current (I d * 1 , I q * 1 ) by referring to a relationship among the voltage (V dc ) of the battery (10), the optimum current (I dp , I qp ) and the output state (T, N) of the synchronous motor (14), using the converged voltage (V s ) of the battery (10) and the output state (T, N) of the synchronous motor (14), determines second optimum reference current (I d * 2 , I q * 2 ) in accordance with the optimum current (I d * 2 , I q * 2 ) in a region where the optimum current (I dp , I qp ) does not depend on the voltage (V dc ) of the battery (10), selects either of the first optimum reference current (I d * 1 , I q * 2 ) or the second optimum reference current (I d * 2 , I q * 2 ) on the basis of a relationship between the magnitude of the first optimum reference current (I d * 1 , I q * 1 ,) and that of the second optimum reference current (I d * 2 , I q * 2 ), and determines selected optimum reference current as the reference current (I d *, I q *). The driving and controlling apparatus according to Claim 12 wherein the current condition determination means (16) approximately represents, with a mathematical equation, a relationship between the voltage (V dc ) of the battery (10) and the optimum current (I dp , I qp ) in a region where the values of the optimum current (I dp , I qp ) depends on the voltage (V dc ) of the battery (10), and stores a coefficient (k d , k q , I dmin , I qmin ) of the mathematical equation and the output state (T, N) of the synchronous motor (14) associated therewith; stores the optimum current (I dp , I qp ) in a region where the optimum current (I dp , I qp ) does not depend on the voltage (V dc ) of the battery (10), reads the coefficient (k d , k q , I dmin , I qmin ) of the mathematical equation in accordance with the output state (T, N) of the synchronous motor (14), and computes the first optimum reference current (I d * 1 , I q * 1 ) by the use of the read coefficient (k d , k q , I dmin , I qmin ), the converged voltage (V s ) of the battery (10) and the mathematical equation, and determines the second optimum reference current (I d * 2 and I q * 2 ) by reading the optimum current (I d * 2 , I q * 2 ) in the region where the optimum current (I dp , I qp ) does not depend on the voltage (V dc ) of the battery (10). The driving and controlling apparatus according to Claim 13 wherein the mathematical equation is a predetermined linear equation, and the current condition determination means (16) stores at least one of an intercept (I dmin , I qmin ) or inclination (k d , k q ) of the linear equation as the coefficient associated with the motor output state (T, N). The driving and controlling apparatus according to Claim 14 wherein the intercept (I dmin , I qmin ) of the linear equation is used as the reference current.

Inventors: KAWABATA YASUTOMO, KUSAKA YASUSHI, YAMADA EIJI, KAWABATA, YASUTOMO, KUSAKA, YASUSHI, YAMADA, EIJI, KAWABATA, YASUTOMO, C/O TOYOTA JIDOSHA K.K., KUSAKA, YASUSHI, C/O TOYOTA JIDOSHA K.K., YAMADA, EIJI, C/O TOYOTA JIDOSHA K.K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1994
Main classification: B60L11
IPC codes: B60L15, B60L11